This invention relates to the optical characterization of particulate dispersions, and in particular to a method and apparatus for the in-situ physical analysis of particles present in fluid substances or in air.
There are many applications where particle characterization measurements can provide for improved process control leading to increased throughput, higher recovery rates, reduced reagent consumption and better product quality. These benefits result in reduced cost and increased profits, strong justifications for the use of process control instrumentation. Unfortunately, there is a lack of particle measurement instrumentation that can be used in-situ for real time measurements that are necessary for process control. In a typical process, such as a polymerization or crystallization reaction, particles or droplets are suspended in a flowing medium, liquid or gaseous, while chemical or physical changes are taking place to the materials in the slurry. In many cases these changes are very dynamic, and thus the materials cannot be measured when removed from the pipeline or vessel requiring the instrumentation to be non-invasive.
Of the common approaches to particle size determination, light scattering is one of the most attractive alternatives, which, besides its intrinsic non-invasive and nonpertubative character, has also the potential for developing high performance online instrumentation and sensing procedures. Current optical technologies utilizing light scattering for particulate characterization are based on: turbidity (see U.S. Pat. No. 4,537,507); dynamic light scattering (see U.S. Pat. Nos. 5,155,549 and 5,502,561); or angular resolved light scattering (see U.S. Pat. No. 5,438,408). These methods require substantial sampling and dilution procedures and therefore are not very suitable for on-line process monitoring. Besides, these approaches are intrinsically invasive.
Advances have been made to develop on-line characterization technologies. Optical techniques are usually preferred because they can be non-invasive, inexpensive and reliable. Several techniques that are pertinent to on-line determination of various properties of particles present in fluid substances have been proposed and are based on: diffusive wave spectroscopy (see U.S. Pat. Nos. 5,365,326 and 5,818,583); coherent backscattering (see U.S. Pat. No. 5,063,301); photon density modulation (see U.S. Pat. No. 4,890,920); and, time-resolved measurements (see U.S. Pat. No. 5,740,291). Thus, the non-invasive optical methods have many advantages for particle sizing, but also have one serious limitation. At high particle concentrations, light is scattered from particle to particle, and such so-called multiple scattering results in loss of precision in the optical measurements.
When light strikes the boundary surface separating two media of different optical densities, some of the incident energy is reflected back. This property is referred to as reflectance and by some authors as backscattering from the interface. The techniques used to measure this property fall under the broad definition of reflectometry. This is different from the backscattering of light that undergoes multiple scattering trajectories in particulate media. It is important to realize this distinction between the single backscattering (reflection) of light from an interface and the light backscattered from a system of particles due to a multiple scattering process. Well known instruments that detect the position and strength of one inhomogenity, i.e., single-scattering in the back scattering direction, are those that rely on low-coherence optical interferometry (sometimes called white light interferometry).
The deficiencies in the current optical light scattering approach are due to the fact that they are based on single interactions between interrogating light wave and specific particles. Therefore, the current methods cannot account for multiple scattering effects and are not appropriate for measurements at high volume fractions of particles such as powders. Many of the current techniques are also limited because of the need for sample preparation and because of their typical bistatic (different locations for source and detector) geometry.
The first objective of the present invention is to provide an optical apparatus for system analysis and process control.
The second objective of this invention is to provide a non-invasive means for optical characterization of particulate fluids.
The third objective of this invention is to provide a low-coherence interferometer for determining the optical path length distribution for light reflected from a random medium.
The fourth objective is of this invention is to provide a low-coherence interferometer with multiple measuring heads.
The fifth objective of this invention is to provide a low-coherence interferometer with an optical switch between different measuring heads.
The sixth objective of the invention is to provide a low-coherence interferometer with multiple wavelength sources.
The seventh objective of the invention is to provide a complex system where low-coherence interferometry LCI information can be enhanced and complemented by the use of time-resolved data. In the same basic configuration, the light source(s) can be modulated at high frequencies and phase and/or amplitude of the LCI signal can be monitored. This information will complement the steady state measurement and will offer the possibility to discriminate between absorption at the particle level.
The primary embodiment of the invention provides a low-coherence interferometer apparatus for determining the size characterization of a stream of particulate or colloidal suspension by means of a split beam of electromagnetic radiation illuminating both a sample probe positioned in said stream and a reference probe, the beam reflections of both probes are combined to provide an interference signal and this signal is thereafter analyzed to provide a photon pathlength distribution whereby the size characterization of said stream is determined. The method of the invention includes the steps of illuminating both an in-situ sample of a steam of particulate or colloidal substances and a reference with a common level of low-coherent electromagnetic radiation and thereafter combining the resultant reflected radiation from said sample and said reference to provide an interference signal whereby the photon pathlength distribution of said sample is realized and thus providing indicia determinative of the size characterization of said stream.